Ventilation/Perfusion

Question 1

What are the roles of the respiratory system?

Answer 1

Gas exchange between O2 and CO2
Reservoir for blood and O2
Metabolise some circulating compounds
Filter blood

Question 2

What are the main functional units of the respiratory system?

Answer 2

Alveolar sacs, surrounded by a dense distensible capillary network

Question 3

What makes up the diffusion barrier?

Answer 3

Diffusion through gas to alveolar wall
Epithelial cell of alveolus
Tissue fluid and connective tissue
Epithelial cell of capillary
Plasma
RBC membrane and cytoplasm

Question 4

Diffusion rate equation? (Fick’s law)

Answer 4

Considers area, diffusion constant (solubility and molecular weight), thickness and partial pressure gradient

Question 5

What are the two types of dead space that make up physiological dead space?

Answer 5

Serial = anatomical dead space where there’s no gas exchange (conducting airways volume)

Distributive = no blood flow so no gas exchange (eg damaged alveolus)

Serial + Distributive = Physiological

Question 6

What is minute ventilation?

Answer 6

The amount of air moved into the lungs per minute
Calculated by volume moved per breath (tidal volume) and respiratory rate
MV = Vt x RR (at rest 6-8L/min)

Question 7

What is the alveolar ventilation rate?

Answer 7

Amount of air that moves into the alveoli/min, need to take into account the amount of wasted alveoli space (dead space, Vds)
AVR = (Vt - Vds) x RR

Question 8

What effect does increasing AVR have on diffusion rate?

Answer 8

Will remove more CO2 from alveoli, lowering pCO2 and increasing the pressure gradient which will increase diffusion coefficient (return to equation)

Question 9

What will happen if there’s no ventilation on pCO2 and pO2 in alveoli and capillaries?

Answer 9

Will get to the point where there is no pressure gradient and pO2 is 6kPa and pCO2 is 6.5kPa in both alveoli and capillaries

Question 10

How do the V/Q ratios differ in pneumonia and pulmonary embolus?

Answer 10

V/Q ratio lowered in both.
Pneumonia - ventilation reduced
Pulmonary embolus - perfusion reduced

Question 11

What’s the effect of gravity on ventilation and perfusion?

Answer 11

Perfusion is greater at the base of the lung as it is nearer the heart so doesn’t need to pump up against gravity

Basal lung is more compressed in its resting state so has more potential to expand to increase ventilation at the base
= V/Q almost 1 at the base

Question 12

Why would a V/Q ratio lead to shunting?

Answer 12

Blood perfusing non-ventilated alveoli will lower V/Q ratio nearer to 0 which is the main cause of hypoxaemia and is seem in pneumonia/pulmonary oedema where ventilation is reduced